1. Field of the Invention
This invention relates generally to combustion processes and more particularly to methods and devices for determining the percent stoichiometric oxidant in the pyrolysis section of incinerators.
2. Description of the Prior Art
In incineration applications, it is common practice to employ two stages of combustion. In the first stage, combustion air is supplied at a rate less than the stoichiometric air requirement. The stoichiometric air requirement is defined as the air flow rate required for complete combustion of the fuel and waste streams. Complete combustion means that the products of combustion are stable compounds such as CO2, H2O, N2 and He (if existing).
Thus, in the first stage the wastes are commonly pyrolyzed in an oxygen-deficient atmosphere. This furnace, or portion of the furnace, is commonly referred to as a reduction, primary combustion, oxygen-deficient, or pyrolyzing furnace or chamber. Additional combustion air is then supplied at a subsequent section to destroy any products of incomplete combustion. This secondary section is typically referred to as a re-oxidation section or afterburner.
Pollutant emissions are strongly influenced by the amounts of combustion air supplied to the pyrolyzing section and the afterburner. Therefore, it is highly desirable to be able to measure and control the air supply to both sections. The air supply to the afterburner is typically regulated to achieve a certain level of excess oxygen in the stack exhaust gases, or in some cases to achieve a target temperature. The air, or oxidant, supply to the pyrolyzing section is more difficult to control. It is desirable to measure and control the oxidant supply to the pyrolyzing section as a percent stoichiometric oxidant, or “PSO.” The PSO is equal to the actual oxidant supply divided by the stoichiometric oxidant supply expressed as a percent. Although oxidants include compounds such as NO and NO2, in practice the main source of oxidant for incinerators is generally air. Therefore the term “PSA” (percent stoichiometric air) is often used in place of PSO.
The PSO can also be related to an equivalence ratio. The equivalence ratio is defined as the actual fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio. The equivalence ratio is related to PSO in that the equivalence ratio is simply 100/PSO. Where fuel and air are supplied to achieve complete combustion, the reaction is said to be stoichiometric, the PSO is equal to 100% and the equivalence ratio is equal to 1.
One common means of directly regulating the air supply to the pyrolyzing furnace is to measure the flow rates of fuel, waste, and air; calculate the PSO; and then control the PSO to a certain value by changing the air supply. Waste compositions often vary with time, or are simply unknown. In practice, because of the difficulties associated with the uncertainties and fluctuations in waste compositions, the waste is often excluded from the stoichiometric air requirement calculation. Because of this exclusion, the method cannot accurately reflect the correct air requirement.
Other common methods for controlling the air supply are either measuring and controlling the combustible level in the pyrolyzing furnace or measuring the temperature change due to addition of afterburner air. These methods are indirect ways of controlling the PSO and do not determine the actual PSO or consider the effect of varying temperature on the actual PSO.
Some methods include use of oxygen sensors in the exhaust gas. For example, U.S. Pat. No. 4,459,923 filed in 1983, by F. M. Lewis, describes a method for controlling the operation of a multiple hearth furnace by controlling the temperature of the hottest hearth and maintaining a minimum O2 content of the exhaust gas. The PSA is calculated from the oxygen measurement in the exhaust gases using the equation:PSA=[1+% O2/(21−% O2)]×100Of course, this relationship is only useful if the PSA is greater than 100% since the oxygen concentration cannot be negative or greater than 21% in the exhaust gases when ambient air (rather than pure oxygen) is used; this expression cannot, and is not intended to, produce a result less than 100%. Thus the relationship requires a fuel-lean or super-stoichiometric combustion and is not applicable to fuel-rich or sub-stoichiometric combustion wherein the oxygen level becomes very low (such as ppm or even ppb level). Indeed, application of this equation in a fuel-rich combustion will result in an erroneous conclusion that the PSA is equal to 100% when it should actually be much less than 100%. Additionally, while maintenance of a constant temperature within certain O2 measurement boundaries provides a means of control, these prior art control methods are not based on the actual PSA. Generally, the control approach has been to maintain a constant temperature rather that a constant PSA, and there has been no attempt to calculate the actual PSA variations due to changes in temperature.
Oxygen sensors have also been used to measure the air/fuel ratio, or equivalence ratio, in internal combustion engines and such devices have been widely used in automobiles (see, for example, U.S. Pat. No. 4,283,256 filed in 1980, by Howard and Wheetman). However, these sensors do not take into account the dependency of equivalence ratio on oxygen level and temperature and therefore cannot operate in wide ranges of temperatures. Fortunately, such devices are able to neglect the effect of temperature on predictions of the equivalence ratio because the exhaust gas temperatures are normally regulated within a relatively narrow range.
Still other devices have been developed due to the recognized need to account for the effects of temperature. For example, U.S. Pat. Nos. 4,151,503 and 4,391,691 utilize semiconductor chips processed to exhibit a rapid change in electrical resistance responsive to differences in exhaust gas temperature. The temperature-dependent electrical resistance is used to compensate the signal from the oxygen sensor to produce a more accurate prediction of the PSO. Due to the mechanical and electrical characteristics of the materials used in the temperature-compensating chips, such devices cannot be operated in the high temperatures (1400° to 3200° F.) commonly seen in the pyrolyzing sections of incinerators.
Thus, there are needs for methods to directly measure the PSO in pyrolosis sections of incinerators that compensate for temperature fluctuations and that avoid the problems described above.